This invention pertains to an improved process for hydroformylating an olefinically-unsaturated compound to produce a mixture of aldehyde products.
It is well known in the art that one or more aldehyde products can be produced by contacting an olefin with carbon monoxide and hydrogen under reaction conditions in the presence of a metal-organophosphorus ligand complex catalyst. One such process, as exemplified in U.S. Pat. Nos. 4,148,830, 4,717,775, and 4,769,498, involves continuous hydroformylation with recycle of a solution containing the metal-organophosphorus ligand complex catalyst, with rhodium being an example of a suitable metal. The mixture produced typically includes linear, or normal, and branched aldehyde products. In some economic conditions, a high normal to branched (normal/branched or N/I) isomer ratio of the product aldehydes is desired.
The N/I ratio of a rhodium-catalyzed hydroformylation is determined primarily by the ligand employed. Because it is inexpensive, and gives rise to a relatively active and selective catalyst, triphenylphosphine (TPP) is often used for industrial hydroformylation processes. Although the rhodium/TPP catalyst is successfully practiced in facilities worldwide, it is limited to about a 10:1 ratio of normal to iso-aldehyde product. Normal aldehydes often have a higher value in the marketplace, thus it would be desirable for those currently operating rhodium/TPP-based processes to be able to increase their production of normal aldehyde in a facile, cost-effective manner.
In a rhodium/TPP system, the number of TPP molecules coordinated to rhodium is proportional to the concentration of TPP employed. At low concentration (e.g. 5-10 moles TPP per mole of rhodium), most rhodium complexes will contain only one TPP molecule. Such complexes are quite active, but generate a low N/I product. Because the selectivity of a rhodium/TPP catalyst increases as the concentration of TPP increases, commercial rhodium/TPP systems are typically operated with a large excess of TPP (e.g. 100-200 moles per mole of rhodium).
WO 2009/035204 teaches increasing the N/I produced by a rhodium/TPP hydroformylation catalyst via the addition of an additional phosphine ligand and a phosphine oxide. However, WO 2009/035204 only demonstrates the enhancement for TPP levels of 50 to 60 moles per mole of rhodium. Because it is well known that the concentration of TPP directly affects the nature of the TPP/rhodium complex, the skilled person would anticipate that a catalyst system containing commercial levels of TPP (100-200 moles TPP per mole of rhodium) would behave quite differently than one containing only 60 moles of TPP. Moreover a simpler process where one could increase the N/I by adding a single additional component would be desirable.
Bisphosphites are known to be active and selective ligands for rhodium hydroformylation. However, EP 0839787 indicates that a rhodium/bisphosphite complex would be vulnerable to additional coordination by TPP. This would result in low-activity, tri-phosphorous complexes. Therefore, the skilled person would expect that bisphosphites would not be capable of increasing the N/I of a rhodium/TPP catalyst employing commercial levels of TPP ligand without a critical loss of reaction rate.